The route of administration is an important part of the product characteristics of a peptide-containing product. An injection, tablet, oral solution, or cream applied to the skin can lead to a completely different level of exposure of the body to the substance, a different rate of absorption, and the penetration of the substance into other tissues. Even if the active ingredient’s name is the same, the results obtained for one formulation or route of administration cannot be automatically transferred to another.
In the case of peptides, this is of particular importance because many of these molecules encounter significant biological barriers. Digestion, enzymatic degradation, tissue permeability, formulation stability, absorption, distribution, and elimination can all influence the amount of substance that ultimately reaches the bloodstream or a specific tissue.
Some peptide drugs have been developed as oral or nasal forms. However, this does not mean that a similar solution will be effective for all peptides. Success depends on the properties of the specific molecule, the formulation, the delivery technology used, and the clinical evidence regarding the finished product.
Studies of oral semaglutide, oral octreotide, and porcine glucagon demonstrate several different ways to solve the problem of delivering peptides.[1][2][3] Each of these examples concerns a specially formulated formulation studied for a specific purpose. None of them demonstrate that any peptide can simply be transferred from a parenteral formulation to a tablet, a spray, a cream, or another form and expect the same exposure, efficacy, or safety profile.
Therefore, when comparing the routes of administration, it is more important than the question „can this peptide be administered in a specific way?” to determine which formulation was actually studied, how exposure was measured, what endpoint was evaluated, and whether the product claimed to be used in the study corresponds to the product used in the study.
The article discusses the scientific differences between peptide delivery routes and how to interpret the studies related to them. It does not include instructions for performing injections, reconstituting, handling equipment, converting doses between delivery routes, or preparing or administering peptide products. Regulatory examples apply to specific jurisdictions and were reviewed on September 20, 2026.
Injections, oral administration, transudative and local administration
The terms „injection,” „oral,” „parenteral,” and „local” describe the route of administration, but even within each of these categories there can be very different formulations and exposure profiles.
The route of administration is therefore only one of the elements of the characteristics of the finished product. In itself, it still does not explain how a given formulation will behave in the body.
The injection is not a single, uniform route of administration
The word „injection” may suggest a single standard method of administration, but in reality it encompasses several different routes of administration.
Intravenous administration introduces the preparation directly into the bloodstream. Subcutaneous administration means placing it in the tissue beneath the skin, from where absorption occurs. In the case of intramuscular administration, the preparation reaches the muscle tissue, which creates additional conditions for absorption.
The formulation itself also influences the further behavior of the substance.
Some injectable preparations are formulated so that the substance is available relatively quickly, while others are designed to modify its release over time. Therefore, even two injectable preparations containing the same active ingredient do not necessarily have identical concentration profiles over time.
Circumventing the gastrointestinal tract eliminates one of the important barriers, but does not eliminate the other biological processes.
After administration, the peptide may still undergo enzymatic degradation, be distributed to various tissues, interact with receptors, be metabolized, and eliminated by the kidneys, liver, or other mechanisms. It may also cause adverse effects related to its own pharmacological activity.
The injection also introduces additional issues related to the formulation quality, sterility, product compliance with the device, the operation of the dispensing system, and the integrity of the product itself.
Therefore, the study regarding intravenous infusion should not be described simply as evidence regarding „peptidic injections”. The significance lies in the specific route of administration, formulation, population studied, exposure, and endpoint evaluated.
This does not mean that all peptide injections are pharmacokinetics the same. Intravenous, subcutaneous, and intramuscular administration involves different processes, and the formulation can additionally alter the rate of release and absorption.
Oral administration involves digestive barriers and a problem with absorption
The peptide taken orally enters a completely different biological environment.
Before reaching the general circulation, it may encounter the acidic environment of the stomach, digestive and intestinal enzymes, mucus, the epithelial barrier, and additional metabolic processes.
These factors can alter or break down peptide molecules before a significant amount of them reaches the bloodstream.
The next challenge is getting through the barrier of the digestive tract. The extent to which this can occur depends both on the properties of the specific peptide and on the formulation in which it was administered.
An excellent example is oral semaglutide.
This peptide was developed together with the SNAC absorption enhancer. Preclinical and clinical studies indicated a formulation-dependent local mechanism of absorption in the stomach.[1]
The most important conclusion therefore does not simply read: „peptides can be taken orally”.
The study shows rather that a properly designed formulation can, in the case of a specific molecule, help overcome specific delivery barriers.
Therefore, a conventional capsule containing a peptide powder is not the equivalent of the semaglutide tablet under review.
However, one should not fall into the other extreme and assume that every orally ingested peptide must be completely ineffective. After consuming certain products, small peptide fragments derived from the food can be detected in the bloodstream.
The conclusion should therefore depend on the specific molecule and formulation: the exposure after oral administration should be demonstrated for the product under investigation, and it should not be assumed solely on the basis of the route of administration.
This difference is particularly important also when interpreting information about Peptide supplements, their forms, composition, absorption and scientific evidence. The presence of the peptide in the oral product does not yet answer the question of what part of the material and in what form it can be biologically available.
Can peptide drugs work after oral administration?
Yes, there are specific drugs and peptide formulations for which clinically significant activity has been demonstrated after oral administration. The EMA lists, among others, the semaglutide oral formulation that is approved for marketing, and the oral octreotide has been studied in patients with acromegaly. [6] [2] However, these are specially designed and tested products. This does not mean that any powder peptide becomes clinically effective just because it is swallowed.
Does the absorption enhancer work with every peptide?
This cannot be assumed. In studies with oral semaglutide, the mechanism related to SNAC was dependent on the specific formulation and substance. [1] Another peptide, another formulation, or another technology that increases absorption requires its own studies. The term „increased absorption” alone does not replace measurements performed for the actual product.
The effective administration of peptides requires its own evidence
Diagnostic products interact with the tissues of the nasal cavity and can be designed for local or systemic action.
Their properties depend on the molecule, formulation, delivery device, site of administration, absorption characteristics, and the intended clinical effect.
A good example is a randomized crossover study in which a specific formulation of glucagon was compared with glucagon administered intramuscularly during experimentally induced hypoglycemia in 75 adult patients with type 1 diabetes. The researchers evaluated the glucose concentration recovery and adverse effects under controlled conditions.[3]
This is direct evidence regarding specific formulations, the population being studied, and the outcome being evaluated.
However, this does not mean that every peptide administered through the nose will achieve clinically useful systemic exposure, nor that every oral formulation will be equivalent to an injection.
It is also particularly important that reports of a whistleblower should not be automatically equated with the direct delivery of the substance to the brain.
The prodrug can cause systemic effects through absorption into the bloodstream, without demonstrating significant delivery of the substance to a specific neurological target.
Claims regarding central nervous system exposure require studies directly related to this topic. This is particularly important when interpreting broader claims regarding the effects of electromagnetic fields on the human body. peptides, sleep, memory and brain function.
Does the peptide administered orally automatically reach the brain directly?
No. The reportable dose can lead to an overall effect without demonstrating direct transport to the brain. The study of the reportable dose evaluated the return of hyperglycemia after hypoglycemia, not the delivery of the substance to specific neurological structures. [3] Therefore, the claim of delivery to the brain requires evidence regarding that specific target site.
Local and subcutaneous administration do not mean the same thing
The terms „local” and „transcutaneous” are sometimes used interchangeably, but from a scientific point of view they describe different concepts.
Local application usually means applying the product to the surface of the body, most often on the skin.
Such a product can be designed to produce a local effect at the site of application or in its immediate vicinity.
Transdermal delivery, on the other hand, refers to the passage of a substance through the skin in order to achieve systemic exposure.
Therefore, a product applied topically can cause measurable local changes without simultaneously delivering a clinically significant amount of the active substance into the bloodstream.
A small randomized study of a cream containing tetrapeptide-68 illustrates this difference well. In 25 women, parameters related to wrinkles around the eyes were evaluated over 12 weeks. [4]
Such a study can provide information about the complete formulation applied locally and about the local endpoints in the area under study.
However, it does not demonstrate systemic exposure to the peptide.
It also does not confirm the activity of an injection preparation containing the same or a similar peptide.
It would also be problematic to reason in the opposite direction. The fact that a peptide administered by injection produces a specific systemic effect does not prove that placing it in a cream will allow the appropriate amount of the molecule to penetrate through intact skin and recreate that effect.
Skin penetration and systemic exposure require their own evidence.
Can the study of a peptide administered by injection confirm the action of the topical preparation?
It can provide biological background and justification for further studies, but it does not constitute direct evidence of the efficacy of a topical preparation. The study of a tetrpeptide applied topically concerned a specific cream and local parameters related to wrinkles. [4] Data regarding systemic administration do not replace studies showing that a specific topical formulation delivers the substance appropriately and affects the declared local endpoint.
The name of the format may hide the actual path to the submission.
Terms such as spray, serum, pen, patch, vial or ampoule describe the form of the product, the packaging or the way it is presented. They are not full pharmacological categories.
The spray can be applied to the nose, skin, hair, or any other area.
The pen can be a device used to administer a medicinal product by injection.
The serum can simply be a cosmetic formulation applied topically.
The ampoule describes the packaging and does not itself state whether its contents are a medicinal product, a cosmetic product, an injection preparation, or a product intended for another use.
The actual route and the place of administration should therefore be determined based on reliable information regarding the specific product.
Additional confusion can be created by marketing language. Terms such as „liquid needles” are sometimes used metaphorically in cosmetics, but they do not indicate exposure similar to injections.
The metaphor cannot demonstrate penetration through the skin, systemic absorption, pharmacokinetics, or equivalence with the injectable formulation. Real-world formulation studies are needed to answer these questions.
The comparison of the routes of administration begins with the correct scientific question.
Each route of administration raises other questions regarding exposure.
In the case of intravenous administration, what happens after the substance is directly introduced into the bloodstream is the most important factor. Data from intravenous administration do not automatically allow for determining what will happen after absorption through the skin, the digestive tract, muscles, or the mucous membrane of the nose.
In the case of subcutaneous and intramuscular administration, it is also important how the preparation leaves the site of administration and enters the bloodstream. Even within these routes, identical concentration profiles over time should not be assumed for all formulations.
In the case of oral administration, the digestion, stability, absorption in the stomach or intestines, and which intact molecules or metabolites ultimately reach the bloodstream must be analyzed.
When reporting a study, they should specify the local or systemic exposure produced by a specific formulation. The reporting route itself does not prove equivalence to injection or direct transport to the brain.
In the case of topical products, the effect of the complete formulation on the intended local endpoint must be demonstrated, and if systemic action is declared—the actual transdermal exposure must also be demonstrated.
In the case of pens and other devices, the first question should be what product and through what route the device actually delivers. The device's appearance alone does not confirm its composition, quality, or regulatory status.
These differences do not form a simple ranking from „best” to „worst” route of transmission. Rather, they show that each of them requires answering different scientific questions.
How the route of administration affects exposure to the peptide
The route by which a peptide enters the body can significantly influence the amount of substance reaching the bloodstream, the rate at which exposure occurs, its duration, and the tissues that may be exposed to the substance.
Therefore, the amount itself given on the product label does not allow for predicting the biological exposure.
The quantity stated does not have to correspond to the quantity reaching the destination.
The exhibition describes how much of the substance reaches a specific biological range and how this amount changes over time.
The nominal amount present in a tablet, capsule, oral preparation, topical product or injection product by itself does not determine this exposure.
In the case of routes other than intravenous absorption may limit or delay the entry of the substance into the bloodstream.
After absorption, the amount of available substance and the site of its distribution are influenced by further processes, including distribution in the body and elimination.
Therefore, the same chemical name placed on two labels does not prove equivalent exposure.
Even identical nominal amounts administered by different routes do not necessarily lead to the same concentrations in the blood or tissues.
The total exposure, maximum concentration and time may vary
Pharmacokinetics uses several parameters to describe different aspects of exposure.
The curve-based concentration-time ratio, or AUC, summarizes the measured systemic exposure over a given time period.
Cmax means the highest measured concentration.
Tmax describes the time required to reach this maximum.
The duration of the exposure may also be important, because some biological effects may depend on how long the concentration remains within a certain range.
These parameters are not interchangeable.
Two formulations can provide similar total exposure, but different maximum concentrations. They can reach similar peak levels at different times or cause different durations of substance retention.
Furthermore, clinical effects may depend on receptor activity, tissue distribution, active metabolites, or concentrations in a specific tissue, rather than solely on plasma concentrations.
These concepts explain why direct comparative studies are necessary. They should not be interpreted as a basis for independently converting one reporting method to another.
The detection of the substance should distinguish the original peptide from its fragments.
The method of analysis is of great importance for the interpretation of the exposure-related claims.
The laboratory method can detect an intact active molecule, a metabolite, several related fragments, or a subsequent biological response. These are not equivalent measurements.
In studies concerning collagen, for example, certain peptides containing hydroxyproline were detected in the blood of humans after the consumption of gelatin hydrolysate.[5]
This shows that the measured small peptide fragments were present in the bloodstream.
However, this does not mean that all the peptides originally present in the hydrolase have been transported into the blood in an intact form.
Therefore, if the claim concerns systemic exposure to a specific intact peptide, the analytical method should appropriately distinguish it from degradation products, metabolites, and related compounds.
The formulation can be just as important as the route of administration
A peptide product is more than just the active ingredient itself.
Chemical modifications, excipients, ingredients that enhance absorption, protective matrices, coatings, release systems, stabilizers, and other formulation elements can all influence exposure.
Studies of oral semaglutide show this particularly clearly. The absorption mechanism depended on the specific formulation and was described as being specific to the substance.[1]
This result cannot be transferred to another peptide just because the other product also declares the presence of an absorption enhancer.
The delivery technology should be treated as a hypothesis that requires confirmation until appropriate studies show how it behaves with a specific molecule and formulation.
The technical feasibility demonstrated in the laboratory, the pharmacokinetic exposure in humans, and the clinical effect represent different levels of evidence.
Direct comparison of the delivery routes requires specific endpoints
A study comparing glucagon administered subcutaneously and intramuscularly shows why the endpoint is important.
The pre-determined glycemic response criterion was achieved during 98.7% visits using the transdermal preparation and during all visits using the subcutaneous preparation. The average time to reach the criterion was different, and discomfort in the head or face occurred more frequently with the transdermal preparation. [3]
Two formulations can therefore achieve comparable results according to a previously defined criterion, while differing in terms of action time, tolerance, pharmacokinetics, convenience, or other characteristics.
Therefore, it should not be simplified to conclude that the circulating and the subcutaneous glucagon are identical.
The non-inferiority study results refer to a specific population, a defined margin of error, a final endpoint, and statistical analysis. They do not imply equivalence in every respect.
Greater exposure does not always mean a better result
In some situations, greater exposure may be important in the development of a drug if too little of the substance limits its effectiveness.
However, this does not mean that greater exposure is always beneficial.
Increasing systemic exposure may also increase the risk of adverse reactions resulting from pharmacological activity.
Whether a particular exposure profile is beneficial depends on the specific clinical problem and the balance of benefits and risks.
A good example is the study of oral octreotide. In a multicenter study, patients with acromegaly who had previously received somatostatin receptor ligands by injection participated. The study evaluated the maintenance of biochemical control after switching to the oral formulation during the study. Some participants discontinued treatment due to therapy failure or adverse effects.[2]
The study shows that oral administration of the peptide can have clinically significant effects under certain conditions.
At the same time, it shows why obtaining absorption itself does not end the product evaluation process. The clinical response, tolerability, predictability of action, and selection of patients remain important.
More information about the broader security issue can be found in the related material. Side effects and safety of peptides.
Allowed peptide formulations and research materials
One of the most important distinctions in the discussion about peptides is the difference between a ready-to-market finished medicinal product and a substance that has only been tested in the laboratory or is sold as a research material.
Evidence regarding a specific molecule does not automatically establish the status or usefulness of any product containing that molecule.
Marketing authorization applies to a specific product
The marketing of the medicinal product is subject to the availability of a ready-to-use product and to the specified conditions of its use.
It does not automatically cover all possible formulations containing the same active substance.
The EMA documentation regarding Rybelsus describes the oral medicinal product containing semaglutide that is approved for marketing. [6]
The documentation regarding Baqsimi, however, describes a medicinal product containing glucagon, administered using a transdermal device.[7]
These examples show that peptide drugs can be effectively developed in forms other than injections as well.
However, they do not constitute a confirmation for oral or reported products unrelated to them that use similar ingredient names.
The seller cannot therefore prove the equivalence of his product solely by indicating a permitted drug containing the same active substance.
The formulation, the manufacturer, the product identity, the regulatory documentation, the production standards, and the scientific evidence must correspond to the product to which the claim relates.
More extensive rules regarding regulatory status are discussed in a separate article: Are peptides legal?.
The clinical trial is not a universal study of substitution of administration routes.
The study of oral octreotide shows another limitation.
A selected group of patients with acromegaly was included in the study; during previous treatment with injectable somatostatin receptor ligands, they achieved complete or partial biochemical control.
The study was open-ended and used a comparison with the baseline values. [2]
These features are part of the evidence and should remain visible during its interpretation.
The results allow conclusions to be drawn regarding the subject matter under study, the formulation, and the population. However, they do not constitute a universal rule according to which the oral form of the peptide can replace the injectable formulation in any situation.
Research material and clinical formulation are different objects of evidence
Laboratory material may contain analytical documentation regarding chemical identity, molecular mass, or purity.
Such information can be scientifically valuable.
However, they do not prove on their own that the material is a validated, ready-to-use formulation intended for administration to humans.
Similarly, the term „research use only” does not constitute proof that the product can be used in humans.
The FDA described products labeled as intended for research purposes that were also offered to consumers for human use.[8]
This should not be interpreted as criticism of properly conducted laboratory activities. It merely shows that the product's labeling alone does not determine its status for human use.
The scientific publication regarding the molecule and the certificate describing the specific sample also answer other questions.
The publication may describe biological activity. The certificate may characterize selected analytical properties of a specific batch.
None of these documents alone confirms that the product delivered corresponds to the finished formulation used in the clinical trial.
Prescription drugs require separate interpretation
Prescription-prepared products constitute another category of evidence that should not be automatically equated with approved finished medicines.
The FDA indicates that prescription drugs are not approved by the FDA and do not undergo the same safety, efficacy, and quality evaluation before being introduced to the market as approved products. At the same time, the FDA recognizes that the manufacture of such preparations may meet specific medical needs.[9]
Both elements of this distinction are important.
A prescription product should not be automatically considered identical to an authorised medicinal product just because the active substance name is similar.
From the perspective of evidence interpretation, the specificity of the product remains the most important factor. The study of one industrially produced formulation is not automatically a direct proof of a separately prepared preparation.
Equipment for dispensing and the risk of contamination
Discussions about peptides often focus on the molecule itself, devoting less attention to the finished product and the delivery system.
In the case of injection preparations and products associated with devices, this can overlook important sources of uncertainty.
Pen is a device, not a separate category of peptides
The pen can be part of a system for administering a medicinal product intended for injection.
However, it is not a separate pharmacological category comparable to the oral, transdermal or injectable route of administration.
The FDA framework for compounded products includes the combination of a drug with a device, including ready-to-use delivery systems.[10]
The device can affect the way the product is delivered, as well as its convenience and ease of use.
Its appearance, however, does not tell us what is inside.
Two similar-looking pills may contain different active ingredients, different concentrations, different formulations, or products with a completely different regulatory status.
Professionally designed packaging is also not proof of approval for marketing.
From a scientific point of view, the description should therefore start with the actual medicinal product and the route of administration. Only then, if relevant, can the device be described.
The mere term „pen peptide” provides too little information to conduct a meaningful scientific comparison.
Are peptide powders and peptide injections separate categories of drugs?
No. A pen can be a device used to administer a drug by injection. A scientifically useful description should first indicate the actual product and route of administration, and only then the device. The word „pen” alone does not sufficiently inform about its contents, formulation, regulatory status, or evidence.
Claims regarding devices require appropriate validation
The dispensing device can be designed to improve convenience, repeatability, or ease of use.
Such properties also require appropriate research.
The evaluation of the device may concern accuracy of dosing, compliance with the formulation, reliability, mechanical properties, and usefulness in a specific population.
These are questions other than the pharmacological activity of the molecule itself.
The peptide may exhibit biological activity, while the delivery system may introduce its own technical problems. On the other hand, a professionally designed and convenient device does not prove that the unverified substance contained within it provides clinical benefits.
The complete product must therefore be evaluated in both dimensions.
Pollution is not the same as pharmacological adverse effects
In the safety assessment, the biological effects of the active substance must be separated from the risks arising from the quality of the product or the way it is administered.
The active substance can cause adverse effects through its intended or unintended pharmacological action.
Pollution is another type of threat.
Improper content of the active substance is another problem.
Chemical degradation, aggregation, or preparation-related problems create the next separate categories of risk.
The CDC lists bloodborne infections and outbreaks of bacterial or fungal infections as among the possible consequences of dangerous injection practices.[11]
These are risks associated with the administration process. They do not prove that each peptide by itself causes infections.
In practical safety assessment, it is therefore worth dividing the three questions into three parts: what risk is caused by the molecule itself, what risk is caused by the quality of the prepared product, and what risk is caused by the process of its delivery.
Combining all these problems into a single general term „peptide risk” makes it difficult to understand what the data actually show.
Product testing revealed various types of quality issues
In one targeted study, products containing semaglutide offered by selected illegal online pharmacies were analyzed. The researchers received three purchased products in vials, but three attempts to purchase the pills did not result in the product being delivered. [12]
The analysis of the vials received showed the presence of active substances exceeding the values declared on the labels, as well as the presence of endotoxins [12].
At the same time, the researchers reported that no live microorganisms were detected in the samples analyzed.
These results show why different quality studies cannot be considered interchangeable.
The absence of live microorganisms does not invalidate the endotoxin result, because the microorganism growth study and the endotoxin study address different types of hazards.
The scale of the study should also be taken into account. A small, intentionally selected group of products was analyzed.
This does not allow for determining the frequency of occurrence of quality problems in all peptide products, online pharmacies, or countries.
The study shows, however, that there are different types of non-conformity and that detecting them requires different analytical methods.
Purity, sterility, endotoxins, identity and content are separate parameters
The percentage of chromatographic purity describes the result obtained using a specific analytical method.
It does not independently confirm sterility.
It does not show the absence of endotoxins.
It does not have to confirm the correct amount of active substance.
It itself does not confirm the full molecular identity or clinical usefulness of the finished product.
Similarly, a negative result of the test for the growth of microorganisms does not confirm the chemical identity or the correct content of the active substance.
The study of purchased semaglutide products shows why different tests can reveal different properties in the same sample. [12]
The Certificate of Analysis, or COA, must therefore be interpreted in accordance with its actual scope.
What matters is the methods used, which batch was tested, whether the sample can be linked to the product delivered, and what exact parameters were measured.
A single positive result regarding purity does not replace a full assessment of the product's quality or the safety of its use in humans.
Does a high percentage of purity mean the correct amount of active peptide?
No. Relative and quantitative purity of the active substance content are separate analytical parameters. In a study of semaglutide products purchased online, the amount of active substance was found to exceed the values declared on the labels. [12] The purity result should therefore be interpreted together with the method used and other quantitative data.
Does the lack of growth of microorganisms mean the absence of endotoxins?
No. These are two different measurements. In the study of semaglutide products, endotoxin was detected despite the absence of live microorganisms in the analyzed samples. [12] A negative result from one microbiological test does not substitute for an endotoxin test or a broader quality assessment.
Storage and handling of the product depend on the specific formulation.
Stability also depends on the actual preparation and packaging.
Peptides and their formulations can undergo chemical degradation, aggregation, and other physical changes depending on their composition and environmental conditions.
The product's appearance alone does not allow us to determine whether it still meets the required parameters.
The CDC notes that the method of preparation can affect both sterility and physical or chemical stability.[11]
The broader scientific conclusion is that the evidence regarding the molecule itself does not eliminate the problems associated with the specific formulation.
The requirements regarding storage and handling should therefore be based on reliable information regarding a precisely defined and appropriately characterized product.
The universal internet rule, which purportedly applies to „all vials containing peptides,” would ignore potentially significant differences between formulations.
Why peptide administration routes cannot be treated interchangeably
The fact that two products contain a molecule with the same name is only the beginning of the comparison.
The route of administration, formulation, exposure profile, population, endpoint of the study, device, and observation period can influence the extent to which results can be transferred from one product to another.
The very name of the molecule is just the beginning
A reliable comparison should take into account the active substance, formulation, route of administration, population, endpoint, and observation period.
If any of these elements actually differ, the evidence may be circumstantial.
An overall study cannot directly confirm the efficacy of a topical product unless its penetration through the skin and the appropriate local endpoint have been examined.
The oral study does not specify the concentration profile over time for the reported formulation.
The intravenous administration experiment does not automatically determine the exposure after subcutaneous administration.
A biologically plausible mechanism may justify starting research on a new formulation, but it does not replace the missing measurements.
This does not mean that every product modification always requires an identical research program from the beginning. It does mean, however, that transferring the findings requires an appropriate evidence bridge, and not simply a common name for the component.
Local results should remain local results
A local product study can provide valuable information regarding wrinkles, hydration, skin appearance, or local tolerance, without simultaneously showing systemic effects.
The study of tetrapeptide-68 is an example of a relatively small study of a specific formulation in a specific anatomical area and over a specific observation period.[4]
The absence of observed reactions in a small study does not confirm the full safety of systemic effects or rule out rare local reactions.
The principle works in the other direction as well.
Adverse effects associated with systemic exposure should not be automatically attributed with the same frequency of local formulation if a comparable systemic exposure has not been demonstrated for it.
The yield preparation does not automatically mean a product targeted at the brain
The route of administration is often presented as if its use alone proves that the substance directly reaches the brain.
Such a conclusion does not stem from the route itself.
The prodrug can cause clinically significant systemic effects.
The study of glucagon evaluated the return of glucose levels after administration of the substance intravenously. [3] The direct delivery of glucagon to the target organ was not studied.
Claims about brain exposure require studies designed specifically to assess such a target site.
Blood concentrations, hormonal response, behavioral change, and direct measurement of the substance in the central nervous system are different types of evidence.
Comparing formulations requires appropriate results
A good study comparing the routes of administration clearly specifies the formulations being studied, the population, the exposure measurements, if necessary, the clinical endpoints, the adverse effects, and the missing data.
Depending on the research question, the device's parameters and its usefulness can also be important.
Different research projects address different questions.
Open-label study of treatment change, randomized cross-over study, pharmacokinetic comparison, and controlled placebo study of a topical preparation are not equivalent types of evidence.
Each of them can provide valuable information, but leaves other areas of uncertainty.
In practice, therefore, it is necessary to assess the specific delivery system in relation to its intended purpose.
Injections should not be considered a universal indicator of greater effectiveness.
Similarly, the form that does not require an injection is not automatically an indication of lower risk.
Most common questions about the routes of administration of peptides
Does bypassing the digestive system make the peptide safe?
No. Skipping digestion removes one of the barriers affecting exposure, but it does not eliminate pharmacological adverse effects, quality issues, impurities, or other hazards. When evaluating safety, one must still consider the specific substance, formulation, and product.
Does the same peptide name mean the same exposure?
No. The formulation and route of administration can significantly influence how much intact substance enters the bloodstream, how quickly it appears there, and how long the exposure persists. Studies on oral semaglutide were based on a specific formulation of the tablet.[1] Therefore, the name of the component is important for identifying the substance, but it does not prove equivalence in pharmacokinetic or clinical terms.
Does the certificate of the research material confirm its suitability for injection?
No. Analytical characterization can provide valuable information about the sample, but by itself it does not prove that the material constitutes a validated finished formulation intended for human administration. The content of active substance, endotoxins, microbiological contamination, and other properties require separate evaluations.[12]
Are peptide preparations that do not require injections free from side effects?
No. The absence of a needle does not eliminate adverse reactions associated with the active substance itself or with the other components of the formulation. The EMA information on glucagon-like peptide 1 describes, for example, both local reactions and systemic effects. [7] The risk should therefore be assessed for a specific product, and not solely based on the method of administration.
Does the bottle with the spray indicate the route of administration?
No. The spray describes the way to administer the product, not its application site. Products in a spray can be intended for the nose, skin, hair, mouth, or other areas. To determine the appropriate route of administration, reliable information about the specific product is needed.
Does the appearance of the pen confirm that the product is authorized for sale?
No. The appearance of the device does not confirm the authenticity of its contents or its regulatory status. The product may resemble a legitimate system that is being supplied, but its identity, manufacturer, content, and legal status must be determined independently.
Does a successful study comparing two routes of administration mean that the products are identical?
No. The study may show comparable results according to one previously defined criterion, while revealing differences in action time, tolerance, pharmacokinetics, convenience, or other properties. The result should be interpreted within the limits of the endpoint, population, and formulation used in the study.
What evidence is needed to transfer the claim regarding the peptide from one route of administration to another?
An appropriate bridging evidence between the actual formulations and the claim to be transferred is needed. Depending on the question, this may include pharmacokinetic data, clinical results, safety information, formulation characteristics, or device testing. The common name of a peptide alone is not sufficient to demonstrate equivalence.
Limitations of evidence regarding the comparison of peptide routes of administration
The first important limitation is the wide range of categories of indication. An injection is not a standard type of exposure, just as an oral, transdermal or topical product does not mean a standard formulation. The evidence should be linked to the specific investigational product.
The second limitation is the dependence on the formulation. The example of oral semaglutide shows that a technology that increases absorption can fundamentally affect the delivery of the substance.[1] However, the success of one formulation does not mean that a similar strategy will work with other peptides.
The third limitation concerns the choice of the endpoint. A similar result according to one of the predetermined clinical criteria does not mean identical pharmacokinetics, activity rate, tolerance, or usefulness. The study of the circulating and intramuscular glucagon shows this difference well.[3]
The fourth limitation is the specific nature of the population being studied. The oral octreotide studies included a selected group of patients with acromegaly who had previously received therapy in injections. [2] These results should not be used as a universal guideline for switching between routes of administration.
The fifth limitation is the difference between local and systemic evidence. A study of a local product may confirm a local effect without demonstrating systemic exposure, whereas a systemic study does not demonstrate that the local product penetrates the skin in quantities sufficient to reproduce such an effect.[4]
The sixth limitation is the interpretation of analytical results. The detection of a peptide fragment, metabolite, or biological response does not always mean demonstrating exposure to the intact active molecule.[5]
The seventh limitation is the identity and quality of the product. Evidence regarding the approved drug cannot be automatically transferred to the research material, prescription preparation, or online product containing a similarly named substance. [6] [7] [8] [9]
The eighth limitation is the multidimensionality of quality. Purity, identity, the content of the active substance, sterility, endotoxins, degradation and stability are different properties. One favorable analytical result does not confirm all of them.[12]
The available data also do not justify creating a universal ranking of peptide delivery routes from the most effective to the least effective or from the safest to the most risky. The usefulness of a particular route depends on the molecule, formulation, intended use, exposure profile, available evidence, and an assessment of the benefits and risks for a particular product.
Summary
The route of administration is not a minor detail regarding the product packaging. It constitutes an important part of its scientific and clinical characteristics.
Administration by injection, oral, transdermal, and topical routes exposes peptides to various biological barriers and may lead to different time-dependent concentration profiles, tissue exposures, clinical effects, and risks.
Even the injection itself is not a standard category. Intravenous, subcutaneous, and intramuscular administration involves different exposure processes, and the formulation can additionally influence the release and absorption.
Oral delivery of peptides particularly highlights the importance of formulation. Studies with semaglutide indicate that effective oral administration may depend on the specialized technology and absorption mechanism appropriate for the specific molecule.[1]
Studies of the reportable glucagon show another important principle: two routes of administration can achieve comparable results according to a specific clinical endpoint, while differing in the speed of action and tolerance. [3]
Studies of topical products, however, remind us that local and systemic effects should remain separate. A cream may affect measurable skin parameters without showing systemic exposure to the peptide.[4]
The same caution applies to authorized medicinal products, research materials, prescription preparations, and dispensing devices.
Evidence regarding the approved product belongs to the specific formulation and route of administration. [6] [7] Research terminology, information about analytical purity, or the familiar appearance of the device are not sufficient to demonstrate equivalence to such a product.
Quality also has to be considered on several levels. Chemical purity, the content of active substances, sterility, the presence of endotoxins, stability, and the performance of the device are not the same thing.[11][12]
Therefore, statements such as „injections work better,” „progenitors peptides reach the brain directly,” „peptides applied to the skin work the same as injections,” „oral peptides cannot work” or „the same peptide works the same regardless of the route of administration” are too broad to be considered as conclusions based on evidence.
The scientifically most justified approach is specific to a particular product.
The active molecule, formulation, route of administration, device, if relevant, pharmacokinetic exposure, clinical endpoint, study population, safety data, and regulatory status should be jointly analyzed.
The common name of a peptide can be a good reason to ask whether the two formulations are comparable.
However, the name itself cannot provide the answer.
Disclaimer
The article is purely educational and scientifically informative in nature. It does not constitute medical advice, recommendations regarding the prescription of medications, the administration of injections, reconstitution, conversion of doses between administration routes, handling of equipment, storage, or instructions for the use of any peptide, drug, investigational material, prescription product, injection product, reportable product, oral or topical product.
Dosage forms and formulations should not be considered interchangeable simply because products contain the same or a similarly named active substance. Evidence regarding an authorized medicinal product should not be automatically transferred to research materials, prescription preparations, products not allowed for sale, or products administered by other routes.
Information regarding the administration, storage, handling, and use of the product should come from the appropriate documentation of the specific product and from appropriately qualified healthcare professionals, and not from general internet instructions regarding peptides.
References
- Buckley, S. T., Bækdal, T. A., Vegge, A., Maarbjerg, S. J., Pyke, C., Ahnfelt-Rønne, J., Madsen, K. G., Schéele, S. G., Alanentalo, T., Kirk, R. K., Pedersen, B. L., Skyggebjerg, R. B., Benie, A. J., Strauss, H. M., Wahlund, P. O., Bjerregaard, S., Farkas, E., Fekete, C., Søndergaard, F. L., … Knudsen, L. B. (2018). Transcellular stomach absorption of a derivatized glucagon-like peptide-1 receptor agonist. Science Translational Medicine, 10(467), Article eaar7047. https://doi.org/10.1126/scitranslmed.aar7047
- Melmed, S., Popovic, V., Bidlingmaier, M., Mercado, M., van der Lely, A. J., Biermasz, N., Bolanowski, M., Coculescu, M., Schopohl, J., Racz, K., Glaser, B., Goth, M., Greenman, Y., Trainer, P., Mezosi, E., Shimon, I., Giustina, A., Korbonits, M., Bronstein, M. D., … Strasburger, C. (2015). Safety and efficacy of oral octreotide in acromegaly: Results of a multicenter phase III trial. The Journal of Clinical Endocrinology & Metabolism, 100(4), 1699–1708. https://doi.org/10.1210/jc.2014-4113
- Rickels, M. R., Ruedy, K. J., Foster, N. C., Piché, C. A., Dulude, H., Sherr, J. L., Tamborlane, W. V., Bethin, K. E., DiMeglio, L. A., Wadwa, R. P., Ahmann, A. J., Haller, M. J., Nathan, B. M., Marcovina, S. M., Rampakakis, E., Meng, L., Beck, R. W., & T1D Exchange Intranasal Glucagon Investigators. (2016). Intranasal glucagon for the treatment of insulin-induced hypoglycemia in adults with type 1 diabetes: A randomized crossover noninferiority study. Diabetes Care, 39(2), 264–270. https://doi.org/10.2337/dc15-1498
- Lee, S. G., Kang, S. M., & Kang, H. (2024). Reduction of wrinkles using tetrapeptide-68 contained in an O/W formulation: A randomized double-blind placebo-controlled study. Pharmaceutics, 16(8), Article 987. https://doi.org/10.3390/pharmaceutics16080987
- Iwai, K., Hasegawa, T., Taguchi, Y., Morimatsu, F., Sato, K., Nakamura, Y., Higashi, A., Kido, Y., Nakabo, Y., & Ohtsuki, K. (2005). Identification of food-derived collagen peptides in human blood after oral ingestion of gelatin hydrolysates. Journal of Agricultural and Food Chemistry, 53(16), 6531–6536. https://doi.org/10.1021/jf050206p
- European Medicines Agency. (n.d.). Rybelsus: European public assessment report. Retrieved September 20, 2026, from Rybelsus: European public assessment report
- European Medicines Agency. (n.d.). Baqsimi: European public assessment report. Retrieved September 20, 2026, from Baqsimi: European public assessment report
- U.S. Food and Drug Administration. (n.d.). FDA’s concerns with unapproved GLP-1 drugs used for weight loss. Retrieved September 20, 2026, from FDA’s concerns with unapproved GLP-1 drugs used for weight loss
- U.S. Food and Drug Administration. (n.d.). Understanding the risks of compounded drugs. Retrieved September 20, 2026, from Understanding the risks of compounded drugs
- U.S. Food and Drug Administration. (n.d.). Definition of combination product: Types of combination products. Retrieved September 20, 2026, from Definition of combination product: Types of combination products
- Centers for Disease Control and Prevention. (2024, March 26). Preventing unsafe injection practices. Preventing unsafe injection practices
- Ashraf, A. R., Mackey, T. K., Vida, R. G., Kulcsár, G., Schmidt, J., Balázs, O., Domián, B. M., Li, J., Csákó, I., & Fittler, A. (2024). Multifactor quality and safety analysis of semaglutide products sold by online sellers without a prescription: Market surveillance, content analysis, and product purchase evaluation study. Journal of Medical Internet Research, 26, Article e65440. https://doi.org/10.2196/65440